Many electronic circuits include two mutually-interacting integrated circuit chips. The interactions between the integrated circuit chips is such that the timing of the chips must be phase-synchronized. One example of such an electronic circuit is a computer system employing two cooperating central processing units or microprocessors. The separate integrated circuit chips receive timing signals from the same system clock signal. However, due to variations in internal signal propagation delays between the two chips, the bus timing signals within each chip and, therefore, all timing signals based on the bus timing signals, are not phase synchronized. Accordingly, there is a need to synchronize the bus timing signals of the different integrated circuit chips. Timing signals are synchronized when their rising edges match in time.
Conventional electronic circuits typically adjust one clock with another using partial digital and all-digital phased locked loop circuits. These circuits are real-time phase locked loop circuits and, as such, are complicated, requiring external control signals or substantial control circuitry for operation. The complicated nature of phase locked loop implementations greatly increases the costs of the circuits. Other disadvantages are inherent in phase-locked loop designs. For example, phase-locked loops often utilize a voltage-controlled oscillator (VCO) in the feedback path. Various limitations are associated with analog voltage-controlled oscillators. One problem is a highly nonlinear transformation of voltage to frequency by the VCO which results from nonlinear effects in MOS transistors of the VCO.
Furthermore, a conventional phase-locked loop system often uses a loop filter to obtain frequency stability in the feedback loop. The loop filter usually includes two capacitors and a resistor which are external to the integrated circuit. An integrated circuit normally produces some internal digital switching noise which induces noise signals on VDD and VSS power supply lines. Because the ground reference potential internal to the integrated circuit includes some noise, the ground potential internal to the integrated circuit has a different potential than the ground reference external to the integrated circuit. This voltage difference between the ground potential internal to the integrated circuit and the ground potential of the loop filter components outside the integrated circuit unavoidably introduces some noise, called phase jitter, on the output of the phase-locked loop.
In many applications, performance degradation arising from phase jitter and nonlinear behavior of the voltage-controlled oscillator is too great to overcome using a phase-locked loop circuit. Therefore a new approach is sought.